The present invention relates to thermal printers and in particular to controlling the power delivered to a thermal print head in response to changes in the ambient temperature surrounding the print head such that characters printed over a wide range of ambient temperatures are printed with the same contrast relative to the surrounding paper.
A thermal print head is used to imprint characters on thermal paper. Thermal paper consists of several layers of laminated material. The surface layer is a temperature sensitive material that the thermal print head melts in the form of a character exposing a colored ink layer beneath the melted surface layer. To melt the surface layer of thermal paper the thermal print head is heated. In prior art thermal printers, the thermal print head was heated by applying a constant voltage and hence constant power to the print head. As the ambient temperature of the print head environment varied, the temperature of the thermal print head would vary resulting in a varying contrast between printed characters and the surrounding paper at one ambient temperature as compared to the contrast between printed characters and the surrounding paper at another ambient temperature. The varying contrast is of greater concern when the printing process spans a long time period such as recording process variables over a span of days or weeks, during which a change in the ambient temperature surrounding the thermal print head is likely.
It is an object of the present invention to provide an inexpensive solution to compensate for changes in thermal paper contrast caused by changes in the ambient temperature surrounding the thermal print head.